Anything related to RTCRtpSendParameters (previously RTCRtpEncodingParameters) really.

For example, RTCRtpSender.getParameters is missing and the setParameters seems to be missing (unless that's the send function). And as far as I can tell, you are not supposed to call setParameters without getParameters first, so that's not spec compliant ;)

And of course, the sendEncodings property of the init parameters in addTransceiver is missing. It is required to setup simulcast (and later SVC when we standardize it).